Radar device and a method for suppressing interference in a radar device

ABSTRACT

A radar device includes elements for generating a carrier signal having a carrier frequency f T , elements for generating pulses with a pulse repetition frequency f PW , elements for distributing the carrier signal to a transmission branch and a receiving branch, elements for modulate the carrier signal in the transmission path using the undelayed pulses, elements for modulating the carrier signal in the receiving branch using the delayed pulses and for generating a reference signal, elements for mixing the reference signal in the receiving branch with a received signal and elements for integrating the mixed signal. Elements are provided for binary phase shift keying (BPSK) modulation of the carrier signal and elements are provided for switching the polarity of the received signal. A method for suppressing interference in a radar device is also described.

FIELD OF THE INVENTION

The present invention relates to a radar device including means forgenerating a carrier signal with a carrier frequency f_(T), means forgenerating pulses with a pulse repetition frequency f_(PW), means fordistributing the carrier signal to a transmission branch and a receivingbranch, means for delaying the pulses, means for modulating the carriersignal in the transmission branch using the delayed pulses and forgenerating a reference signal, means for mixing the reference signal inthe receiving branch with a received signal, and means for integratingthe mixed signal. The present invention also relates to a method ofsuppressing the interference in a radar device including the steps:generating a carrier signal having a carrier frequency f_(T), generatingpulses having a pulse repetition frequency f_(PW), distributing thecarrier signal to a transmission branch and a receiving branch,generating the pulses, modulating the carrier signal in the transmissionbranch using the undelayed pulses, modulating the carrier signal in thereceiving branch using the delayed pulses and generating a referencesignal, mixing the reference signal in the receiving branch with areceived signal, and integrating the mixed received signal.

BACKGROUND INFORMATION

Radar devices and methods according to the related art are used, forexample, in short-range sensing systems in motor vehicles. They areused, for example, to prevent accidents or to detect objects in a blindspot of a motor vehicle.

FIG. 1 shows a schematic view of the basic structure of a radar deviceof the related art. A local oscillator (LO) 110 generates a carrierfrequency f_(T). This carrier frequency is distributed by a powerdivider 116 to a transmission branch and a receiving branch. In additionto carrier frequency f_(T), a pulse generator 112 provides a pulserepetition frequency f_(PW) to modulate the carrier frequency. In thetransmission branch, this modulation occurs using switch 120, to whichthe carrier frequency is applied and which is switched with the pulserepetition frequency. The signal thus generated is emitted by atransmitting antenna 136. A modulation also occurs in the receivingbranch. However, the pulses of the pulse repetition frequency aredelayed by a delaying device 118 for the purpose of this modulation.These delayed pulses are used to modulate carrier frequency f_(T) byoperating switch 122, to which the carrier frequency is also applied. Inthis way, a reference signal S_(R) is made available in the receivingbranch. This reference signal is mixed in a mixer 124 with a receivedsignal received via receiving antenna 134. The output signal of mixer124 is supplied to an integrating means 126, for example, a low-passfilter and an amplifier. The signal thus generated is supplied to asignal analyzer and controller 138, preferably after analog/digitalconversion. Signal analyzer and controller 138 now determines the delayof delaying device 118, which is varied between a value Δt_(min) andΔt_(max). For example, the delay may be varied by a microcontroller orby a digital signal processor. It is also conceivable that specialhardware is used for this purpose. If the transit time of the radarpulses, which as a rule is equal to twice the transit time between thetarget and antenna, is identical to the delay, the amplitude of theoutput signal of mixer 126 is at its maximum. A correlation receiver isthus available via which the distance to the target and the radial speedbetween the target and antenna may be inferred from the delay set bycontroller 138. By way of example, FIG. 1 shows only the formation ofthe in-phase (I) signal. The quadrature (Q) signal is formed in ananalogous manner by mixing with the carrier frequency, which is 90° outof phase.

It is basically desirable to suppress interference signals originatingfrom highly varied sources. The use of additional modulation of themicrowave signal to separate the signal components reflected by thetargets from interference signals has already been described. Suchmethods in particular suppress interference by other uncodedtransmitters, broadcast transmitters for example, or noise.

However, radar devices are also subject to noise resulting fromparasitic effects which are essentially independent of the effect ofother radar sensors. Thus, for example, switches 120, 122 in FIG. 1 havein reality a finite ratio between the resistances in the off or oncondition R_(off)/R_(on). In addition, undesirable emissions or bridgingof the carrier frequency arise from the local oscillator, for example,to the reference input of the mixer. This means that an approximatelycontinuous leakage signal having the carrier frequency and low amplitudeis transmitted in the transmission pauses between the radar pulses. Thisleakage signal in particular is also present irrespective of the delayset in the reference branch and is mixed with the received signal. As aresult of this and other parasitic effects, an interference signal isreceived in addition from targets located outside the distance range(range gate) momentarily set by the delay in the reference signal. Ifsuch “undesirable” targets have a large backscattering cross-section orthey are within short range of the sensor, then the interference signalamplitude may be on the order of magnitude of the desired signalamplitude or exceed it and consequently result in measurement errors.

It is possible to improve the R_(off)/R_(on) ratio and accordinglyreduce the interference signal amplitude by using, for example, severalswitches linked in series. However, this increases the technicalcomplexity and consequently the costs.

SUMMARY OF THE INVENTION

According to a first embodiment, the present invention builds on a radardevice of the related art by providing means for binary phase shiftkeying (BPSK) modulation of the carrier signal. BPSK modulation of thecarrier signal may be used to integrate interference signals withconstantly alternating signs in the subsequent integration while thedesired signal is integrated with a constant sign. The interferencesignals are suppressed in this manner.

According to a second embodiment, the present invention builds on aradar device of the related art by providing means to switch thepolarity of the received signal. In this manner, the subsequentintegration suppresses the interference signals to a great extent whilethe desired signals are further processed.

Preferably, means are provided for BPSK modulation of the carrier signalin the transmission branch. In this variant, the carrier signal in thereceiving branch may be supplied to the mixer as a reference signalwithout BPSK modulation. However, modulation takes place in thereceiving branch so that the information necessary for the interferencesignal suppression is present there.

However, it may also be advantageous to provide means for BPSKmodulation of the carrier signal in the receiving branch. In this case,a BPSK-modulated carrier signal is used as a reference signal while thetransmitted signal is transmitted unmodulated. The information necessaryfor the interference signal suppression is contained in the carriersignal in the receiving branch.

It is useful in particular if the BPSK modulation results in aswitchover of the phase angle for half a period T_(PW) of pulserepetition frequency f_(PW). In this way, the phase of the modulatedcarrier signal is switched between 0° and 180° after each half period.This periodic switchover of the phase angle advantageously ensures thatthe interference signals are integrated with a constantly alternatingsign while the desired signal is integrated with a constant sign.Referring to two periods in each case, a pulse is generated in thetransmission branch in each of the first and second half periods T_(PW)and in the receiving branch in each of the first and fourth halfperiods. The process is repeated after every two periods.

For effective interference signal suppression, it is advantageous inparticular if the mixed signal is integrated over 2n periods T_(PW) ofpulse repetition frequency f_(PW), n being an integer equal to 1, 2, 3,. . . . This ensures that the interference signals are integratedalternately and accordingly suppressed.

It is useful if the ratio between carrier frequency f_(T) and pulserepetition frequency f_(PW) is an integer. This may be attained bydividing the carrier frequency by an integer. Another possibility forhaving the ratio as an integer is to generate the carrier frequency bymultiplying an oscillator frequency with an integer and to generate thepulse repetition frequency by dividing the same oscillator frequency byan integer. The ratio between the carrier frequency and the pulserepetition frequency being an integer provides an effective interferencesignal suppression since the start and end of the pulse always coincidewith a defined phase angle of the carrier signal.

It may also be advantageous to provide means for the BPSK modulation ofthe carrier signal in the transmission branch and in the receivingbranch, to switch the phase angle in the receiving branch as a result ofthe BPSK modulation for a period T_(PW) of pulse repetition frequencyfPW and to switch the phase angle in the transmission branch as a resultof the BPSK modulation in every second pulse period of pulse repetitionfrequency f_(PW) and in the transmission and receiving branch for thelength τ of each pulse. This makes it possible to suppress even externalinterference signals in addition to the interference signals based onparasitic effects.

Furthermore, it may be useful if switching means are provided to switchthe polarity of the received signal. Such hardware-based polarityswitching is suitable for ensuring the integration of the interferencesignal with an alternating sign.

However, it may also be useful if the polarity of the received signal isswitched digitally. Such digital and preferably program-controlledpolarity switching after analog/digital conversion reduces the hardwarecomplexity. The integration in this case is expediently digital, forexample by decimation, i.e., low-pass filtering and subsequent reductionof the sampling rate. In this case, an external low-pass is used tosuppress aliasing. However, with this digital method, the I signal orthe Q signal must be sampled at a high bandwidth B (B>f_(PW)) and acorrespondingly high sampling frequency and further processed digitally.This in turn requires additional hardware complexity.

According to a first embodiment, the present invention builds on themethod of the related art in that binary phase shift keying (BPSK)modulation of the carrier signal occurs. BPSK modulation of the carriersignal may be used to integrate interference signals with a constantlyalternating sign in the subsequent integration while the desired signalis integrated with a constant sign. The interference signals aresuppressed in this manner.

According to a second embodiment, the present invention builds on themethod of the related art in that the polarity of the received signal isreversed. In this manner, the subsequent integration suppresses theinterference signals to a great extent while the desired signals arefurther processed.

Preferably, a BPSK modulation of the carrier signal occurs in thetransmission branch. In this variant, the carrier signal in thereceiving branch may be supplied to the mixer as a reference signalwithout BPSK modulation. However, a modulation takes place in thetransmission branch so that the information necessary for theinterference signal suppression is present there.

However, it may also be advantageous that a BPSK modulation of thecarrier signal occurs in the receiving branch. In this case, aBPSK-modulated carrier signal is used as a reference signal while thetransmitted signal is transmitted unmodulated. The information necessaryfor interference signal suppression is contained in the carrier signalin the receiving branch.

It may also be useful if the BPSK modulation results in a switchover ofthe phase angle for half a period T_(PW) of pulse repetition frequencyf_(PW). In this way, the phase of the modulated carrier signal isswitched between 0° and 180° after each half period. This periodicswitchover of the phase angle advantageously ensures that theinterference signals are integrated with a constantly alternating signwhile the desired signal is integrated with a constant sign.

Preferably, the mixed signal is integrated over 2n periods T_(PW), nbeing an integer equal to 1, 2, 3, . . . of pulse repetition frequencyf_(PW). This ensures that the interference signals are integratedalternately and thus suppressed.

It is useful that the ratio between carrier frequency f_(T) and pulserepetition frequency f_(PW) is an integer. This may be attained bydividing the carrier frequency by an integer. Another possibility forhaving an integer ratio is to generate the carrier frequency bymultiplying an oscillator frequency with an integer and to generate thepulse repetition frequency by dividing the same oscillator frequency byan integer. The ratio between the carrier frequency and the pulserepetition frequency being an integer provides an effective interferencesignal suppression since the start and end of the pulse always coincidewith a defined phase angle of the carrier signal.

Also, it may be advantageous if a BPSK modulation of the carrier signaloccurs in the transmission branch and in the receiving branch, if thephase angle is switched in the receiving branch as a result of the BPSKmodulation for a period of pulse repetition frequency f_(PW) and if thephase angle is switched in the transmission branch as a result of theBPSK modulation in every second pulse period of pulse repetitionfrequency f_(PW) and in the transmission and receiving branch for thelength τ of each pulse. This makes it possible to suppress even externalinterference signals in addition to the interference signals based onparasitic effects. In this embodiment, one pulse is generated in thetransmission branch and one in the receiving branch in each periodT_(PW) of pulse repetition frequency f_(PW).

Preferably the polarity of the received signal is switched by switchingmeans. Such hardware-based polarity switching is suitable to ensure theintegration of the interference signal with an alternating sign.

It may also be advantageous if the polarity of the received signal isswitched digitally. Such digital and preferably program-controlledpolarity switching after analog/digital conversion reduces the hardwarecomplexity. Integration in this case is expediently digital, for exampleby decimation, i.e., low-pass filtering and subsequent reduction of thesampling rate. In this case, an external low-pass is used to suppressaliasing. However, with this digital method, the I signal or the Qsignal must be sampled at a high bandwidth B (B>f_(PW)) and acorrespondingly high sampling frequency and further processed digitally.This in turn requires additional hardware complexity.

The present invention is based on the surprising knowledge that it ispossible to suppress interference by parasitic effects in radar devicesactually constructed with relatively little technical complexity. Theuse of a BPSK modulation or switching the polarity of the receivedsignal makes it possible to integrate the interference signals withconstantly alternating signs while the desired signal is integrated witha constant sign. As an advantageous embodiment in particular, it shouldbe mentioned that it is not only possible to suppress interferencecaused by parasitic effects in both the transmission branch and thereceiving branch, but rather it is also possible to suppress externalinterference signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of a radar device of the relatedart.

FIG. 2 shows a schematic representation of a radar device according tothe present invention.

FIG. 3 shows graphic representations of the amplitudes of transmittedand received signals of the radar device according to FIG. 2.

FIG. 4 shows a schematic representation of a radar device according tothe present invention.

FIG. 5 shows schematic representations of the amplitudes of transmittedand received signals of the radar device according to FIG. 4.

FIG. 6 shows a schematic representation of a radar device according tothe present invention.

FIG. 7 shows a schematic representation of a radar device according tothe present invention.

FIG. 8 shows schematic representations of the amplitudes of transmittedand received signals of the radar device according to FIG. 7.

DETAILED DESCRIPTION

FIG. 2 shows a schematic representation of a radar device according tothe present invention. A local oscillator 10 is connected to a powerdivider 16. This power divider 16 is connected to a transmission branch.One portion of the power of local oscillator 10 is decoupled by powerdivider 16 into the receiving branch. In addition, local oscillator 10is connected to a frequency divider 12. Frequency divider 12 reducescarrier frequency f_(T) of local oscillator 10 by the factor N. Thisreduced frequency is supplied to a pulse shaper 40 and a delay device18. This pulse shaper 40 operates a switch 20 in the transmission branchto modulate the carrier signal. The delayed output signal of delaydevice 18 is sent to a pulse shaper 42. This pulse shaper 42 generatespulses with which the carrier signal in the receiving branch ismodulated using switch 22. A reference signal S_(R) is generated in thismanner. Thus signals are present in both the transmission branch and inthe receiving branch that are modulated with the pulses of therespective pulse shapers 40, 42. The signal in the transmission branchis subsequently supplied to a BPSK modulation 28, which switches thetransmitted signal between 0° and 180°. This switchover between 0° and180° occurs within half a period, which corresponds to period lengthT_(PW) of doubled pulse repetition frequency F_(PW). This isaccomplished in that BPSK modulation 28 is controlled by an outputsignal of frequency divider 12. In this manner, the phase is switchedafter every half period of pulse repetition frequency f_(PW). The BPSKsignal thus modulated is transmitted by transmitting antenna 36 andreceived by receiving antenna 34 after being reflected by a target. Thesignal received by receiving antenna 34 is supplied to a mixer 24 whereit is mixed with reference signal S_(R). The output signal of mixer 24is integrated and amplified in a low-pass filter 26. The output signalof low-pass filter 26 is sent to a signal analysis unit and controller38. Controller 38 sets the delay of delay device 18. The result of theperiodic switchover of the phase angle is that the correspondinginterference signal is integrated with a constantly alternating sign andthe interference signal is thus minimized. However, the desired signalis integrated with a constant sign. For effective interference signalsuppression, it is also necessary that the carrier frequency f_(T) topulse repetition frequency f_(PW) ratio be an integer: N=f_(T)/f_(PW).This may be accomplished, as shown, by dividing the carrier frequency bythe factor N or by multiplying an oscillator frequency f_(O) withf_(T)=N₁f_(O) and dividing f_(PW)=f_(O)/N₂ by N₁N₂=N and N₂=1, 2, . . .(not shown). The formation of the in-phase (I) signal is shown in FIG.2. The quadrature (Q) signal is formed by analogy by mixing with carrierfrequency f_(T), which has been phase shifted by 90°.

The amplitude of transmitted signal S_(T) is shown over two periodsT_(PW) of pulse repetition frequency f_(PW) in the upper portion of FIG.3. Reference signal S_(R) is shown over the same time span in the lowerportion of FIG. 3. FIG. 3 shows the signal relationships that occur in acircuit according to FIG. 2, i.e., with a BPSK modulation in thetransmission branch. It may be seen that the transmitted signal isswitched over after every period T_(PW) of pulse repetition frequencyf_(PW). The delayed received signal is not phase-modulated.

Another radar device is again illustrated by way of example in FIG. 4using the in-phase (I) signal. In this case also, the quadrature (Q)signal is formed in an analogous manner by mixing with carrier frequencyf_(T), which is phase-shifted by 90°. Components corresponding to thesame components in FIG. 2 are identified by the same reference symbols.In contrast to FIG. 2, the receiving branch is BPSK-modulated in theradar device according to FIG. 4. However, the transmitted signals aretransmitted by transmitting antenna 36 without prior BPSK modulation.The BPSK modulation in the receiving branch generates a BPSK-modulatedreference signal S_(R) which is mixed with the received signal in mixer24.

The waveform of the signals that occur in a circuit according to FIG. 4is shown graphically in FIG. 5. Transmitted signal S_(T) is plotted inthe upper portion of FIG. 5. Reference signal S_(R) is shown in thelower portion of FIG. 5. Based on FIG. 5, it may be seen that thetransmitted signal is not phase-modulated. The phase of the delayedreceived signal is switched over after each half period T_(PW) of thepulse repetition frequency.

Another embodiment of a radar device of the present invention is shownin FIG. 6. Again, components that correspond to those of FIG. 2 areidentified with the same reference symbols. The special feature in FIG.6 is that means 32 are provided instead of BPSK modulation for switchingthe polarity of the received signal. This also makes it possible tosuppress the interference based on the subsequent integration since theinterference is integrated with an alternating sign while the desiredsignals are always integrated with the same sign.

An additional embodiment of a radar device of the present invention isshown schematically in FIG. 7. In this case, both the transmissionbranch as well as the receiving branch are provided with BPSK modulation28, 30. BPSK modulation 30 occurs in the receiving branch as a functionof the output signal of an OR gate 44. Input signals of this OR gate 44are the delayed pulses as well as pulse repetition frequency f_(PW)divided by two. In this manner, the phase modulation in the receivingbranch occurs during length τ of the pulses and in every second pulseperiod T_(PW) of the pulse repetition frequency. This, however, requiresgreater hardware complexity.

BPSK modulation 28 in the transmission branch occurs as a function of anadditional OR gate 45. Input signals of this OR gate 45 are the pulsesfrom pulse shaper 40 and pulse repetition frequency f_(PW) divided byfour at the output of an additional divider 46. The phase modulationthus occurs during length τ of the pulses and for the length of twoperiods each of pulse repetition frequency f_(PW) in the third andfourth periods of four periods. After every four periods, the modulationpattern just described is repeated in a similar manner.

The signals that occur in a radar device of the present inventionaccording to FIG. 7 are shown in FIG. 8. Transmitted signal S_(T) isagain shown in the upper portion of FIG. 8. Reference signal S_(R) isshown in the lower part. It may be seen that the phase of thetransmitted signal is switched over after every second period T_(PW) ofthe pulse repetition frequency and during length τ of each pulse. Thephase of the reference signal is, however, switched after each periodT_(PW) of the pulse repetition frequency and during length τ of eachpulse.

The above description of the exemplary embodiments according to thepresent invention is only intended to illustrate and not to limit thepresent invention. Various changes and modifications are possible withinthe scope of the present invention without departing from the scope ofthe present invention and its equivalents.

What is claimed is:
 1. A radar device, comprising: an arrangement for generating a carrier signal having a carrier frequency fT; an arrangement for generating pulses having a pulse repetition frequency fPW; an arrangement for distributing the carrier signal to a transmission branch and a receiving branch; an arrangement for delaying the pulses; an arrangement for modulating the carrier signal in the transmission branch using undelayed ones of the pulses; an arrangement for modulating the carrier signal in the receiving branch using delayed ones of the pulses and for generating a reference signal; an arrangement for mixing the reference signal in the receiving branch with a received signal to produce a mixed signal; an arrangement for integrating the mixed signal; and an arrangement for switching a polarity of the received signal.
 2. The radar device as recited in claim 1, further comprising: an arrangement for performing a binary phase shift keying (BPSK) modulation of the carrier signal in the transmission branch.
 3. The radar device as recited in claim 1, further comprising: an arrangement for performing a binary phase shift keying (BPSK) modulation of the carrier signal in the receiving branch.
 4. The radar device as recited in claim 1, wherein: a phase angle is switched over by an arrangement for performing a binary phase shift keying (BPSK) modulation for half a period TPW of the pulse repetition frequency fPW.
 5. The radar device as recited in claim 1, wherein: the mixed signal is integrated over an integral number of 2n periods TPW of the pulse repetition frequency fPW, where n=1, 2, 3, . . . .
 6. The radar device as recited in claim 1, wherein: referring to two pulse periods TPW in each case, a process is performed in which a pulse is generated in the transmission branch in each of a first half pulse period and a second half pulse period and in the receiving branch in each of the first half pulse period and a third half pulse period, and the process is repeated after every two pulse periods TPW.
 7. The radar device as recited in claim 1, wherein: a ratio of the carrier frequency fT to the pulse repetition frequency fPW is an integer.
 8. The radar device as recited in claim 1, further comprising: an arrangement for performing a binary phase shift keying (BPSK) modulation of the carrier signal in the transmission branch and in the receiving branch, wherein: a phase angle is switched in the transmission branch as a result of the BPSK modulation for two periods TPW each of the pulse repetition frequency fPW and during a length of each pulse, and the phase angle is switched in the receiving branch as a result of the BPSK modulation in every pulse period of the pulse repetition frequency fPW and during the length of each pulse.
 9. The radar device as recited in claim 1, wherein: the mixed signal is integrated over an integral number of 4n periods TPW of pulse repetition frequency fPW, where n=1, 2, 3, . . . .
 10. The radar device as recited in claim 1, wherein: one pulse each is generated in the transmission branch and the receiving branch in each pulse period TPW.
 11. The radar device as recited in claim 1, wherein: the polarity of the received signal is switched digitally.
 12. A method for suppressing interference in a radar device, comprising: generating a carrier signal having a carrier frequency fT; generating pulses having a pulse repetition frequency fPW; distributing the carrier signal to a transmission branch and a receiving branch; delaying the pulses; modulating the carrier signal in the transmission branch using undelayed ones of the pulses; modulating the carrier signal in the receiving branch using delayed ones of the pulses; generating a reference signal; mixing the reference signal in the receiving branch with a received signal; integrating the mixed signal; and switching over a polarity of the received signal.
 13. The method as recited in claim 12, further comprising: performing a binary phase shift keying (BPSK) modulation of the carrier signal in the transmission branch.
 14. The method as recited in claim 12, further comprising: performing a binary phase shift keying (BPSK) modulation of the carrier signal in the receiving branch.
 15. The method as recited in claim 12, further comprising: switching over a phase angle by performing a binary phase shift keying (BPSK) modulation of the carrier signal for half a period TPW of pulse repetition frequency fPW.
 16. The method as recited in claim 12, further comprising: integrating the mixed signal over an integral number of 2n periods TPW of pulse repetition frequency fPW, where n=to 1, 2, 3, . . . .
 17. The method as recited in claim 12, wherein: a ratio of the carrier frequency fT to the pulse repetition frequency fPW is an integer.
 18. The method as recited in claim 12, further comprising: performing a binary phase shift keying (BPSK) modulation of the carrier signal in the transmission branch and in the receiving branch, wherein a phase angle is switched in the transmission branch as a result of the BPSK modulation for two periods of the pulse repetition frequency fPW and during a length of each pulse, and the phase angle is switched in the receiving branch as a result of the BPSK modulation in every second pulse period of the pulse repetition frequency fPW and during the length of each pulse.
 19. The method as recited in claim 12, wherein: the polarity of the received signal is switched over digitally. 